I am playing it a mildly opposed. I.E. It's not against the law in the 7 Pillared Hall, but it gives the town a bad reputation so it needs to be kept to a minimum.

I am also playing the slavers operations in the same way. They are not making their dealings to apparent and the players have to dig for any info they find.

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I made the wizards mildly opposed to slavery.
I placed Thunderspire in the borderlands of Karameikos (Mystara), where slavery is outlawed, and hailing from the academy in Specularum they would continue to hold up the values of their society.
The slavers are therefore working covertly, and although the mages are aware of slave transports going on, as long they happen outside the Hall (and the slavers don't provoke them), they are not going out of their way to start a war.

Indifferent, on the condition that the duergar behave lawfully within the Seven-Pillared Hall. Orontor finds the duergar distasteful as a general principle, but their slaver operation poses no great threat to the Seven-Pillared Hall.

The PCs in my game basically ransacked the duergar shop in the Hall but left no tangible evidence of their guilt. Though Orontor suspected them, he was quite willing to overlook this event, but not without asking a favor in return... a favor for the mages, you might say.

I thought this approach added an interesting overtone to the second half of the module, while linking the storyline in a more coherent way.

Well, in my campaign, slavery and slave trading is forbidden in the hall (as it tends to attract attention), but Orontor simply doesn't care what happens in the wider Labyrinth. The Mages are simply concerned with the Hall being a neutral ground where profitable business may be conducted.

My group found out the Duergar were behind the slavers, and stealthily broke into the trading outpost to find more evidence. Of course, the elf failed a stealth roll, and the party had to fight them (while the halfling backtracked and found the 'paperwork'). Unfortunately, they fought them in the Hall itself, and one of them (again, the elf) refused to stop fighting when commanded by Brugg.

Thus, the Mages, whose word is law in the Hall, were within their rights to execute them for combat, and larceny, within the Hall. However, Orontor offered them an option: Die now, or find Paldemar and bring him back.

If they reenter the Hall, they better either have Paldemar, or information about his whereabouts. If they try to leave Thunderspire through the Hall, the Mages will have them executed. However, to explain why they didn't execute them for larceny and assault in the first place, the Mages made it known that the duergar had flouted the laws of the Hall (by engaging in slave trading in the hall, whether or not the slaves themselves were there). Now everyone thinks the Mages selected this group of heroes to make an example of the Duergar, and reinforce the perception of control over the Hall so important to the Mages.

Plus, now it's not exactly a "favor" for the Mages.

So, to make a long story short, Orontor is opposed to slavery in the Hall, because the Mages told people not to do it, but not for any ethical reasons.

I'm playing Orontor as personally opposed. But in his role as Ordinator his voiced concerns are: non-payment of taxes, the message that sends to the other inhabitants, and the unwanted attention that slave trading has brought to the halls (namely, the adventures and their mission.)

In my Orcus conversion of H2, slavery is not even on the agenda of Orontor, he has more pressing problems. All the Mages of Saruun have gone missing. His focus is elsewhere, so anarchy is starting to break out in the Hall. Might make right - thats why the Bloodreavers and Grimmerzhul can do slavery in public within the halls.

I was running a heavily modified version of Myrhdraak's modification, so what I did went through a couple of filters.

The Mages' pre-existing ban on slavers within their halls, I played like this: "The profits to be had from slavery aren't worth to attendant risks. Short term, certainly, the profit is there, but long term you know some band of heroes is going to show up, or a slave is going to throw off her shackles and say 'No more!' Really, it's just tempting fate..."

So basically, I went with the genre-savvy "we're the lesser of two evils" options, and as the Mages have continued to be, if not allies, then at least recurring employers of my (admittedly very unaligned) party, it's provided any number of opportunities for clever/entertaining snark.